Generally, it is easy to obtain polystyrene beads having a high expansion ratio. The resulting foamed articles made from such beads have high rigidity and good shape retention, but have the disadvantage in that they are fragile and have poor chemical resistance, oil resistance and thermal stability. In pre-expanded bead form, polystyrene has a glass transition temperature of, for example, 80.degree.-95.degree. C., precluding its use in automotive foams, for example, under the hood.
Foamed products of polystyrene and styrene-maleic anhydride are known, e.g., from U.S. Pat. No. 4,442,232 and, although they have higher thermal resistance, they are rather difficult to prepare, and have limitations in their impact resistance and compressive strength.
Expandable thermoplastic beads comprising ethylene-propylene copolymers grafted with vinyl aromatic monomers are also known, e.g., from Kajimura et al., U.S. Pat. No. 4,303,756. The compositions which are produced are said to have excellent thermal stability, but resistance to solvents and oxidation tend to be lower than desirable.
To overcome these drawbacks, it has also been proposed to form foams from blends of polyphenylene ethers and polystyrene or high impact, i.e., rubber-modified grafted, polystyrene imbibed with liquid blowing agents. Mention can be made of U.S. Pat. No. 3,492,249, which suggests foaming a physical blend of polyphenylene ether and polystyrene. However, to develop a maximum strength, the cells have to be elongated and this involves a hot-stretching step, which is not desirable.
In U.S. Pat. No. 4,598,100 and 4,598,101, blends of a polyphenylene ether resin and high impact polystyrene are imbibed at atmospheric pressure with a volatile chlorinated hydrocarbon in an extruder, and the blend is extruded thereafter into a foam.
In U.S. Pat. No. 4,532,263 and 4,598,104, there is disclosed impregnating pellets of a blend of polyphenylene ether and high impact polystyrene, pre-expanding the pellets, and then shaping them in an open mold to form foam. The last-mentioned patent also discloses that foamed sheets of blended polyphenylene ether resin and polystyrene can be thermoformed into shaped foamed articles.
Methods for the production of expandable beads for use in the manufacture of molded articles are well-known in the art. Traditional expanded polystyrene (EPS) technology has existed since the early 1950's.
The production of expandable beads can include the incorporation of a blowing agent by one of the three methods. One method, known as the "one step" process, is to suspend styrene monomer, initiators and hydrocarbon blowing agent in water and polymerize at high temperatures. This method is taught by Japanese Patent Publication No. 32623/70.
In a second process called the "11/2 step" process, the polymerization is brought to 80% completion before the blowing agent is introduced into the system.
The "two-step" process incorporates the blowing agent into suspension-polymerized beads (100% converted) or melt-compounded pellets. The two-step process is used in Japanese Patent Publication No. 36097/72 which discloses heat melting a polymer, pelletizing the melt and incorporating the formed pellets with a blowing agent.
Canadian Patent No. 1,023,521 and U.S. Pat. No. 3,003,193 disclose processes for pellet production wherein the resin is passed through the die and intermittently extruded. Canadian Patent No. 1,023,521 discloses intermittently extruding the resin in a heat plastified form under continuously applied pressure through a plurality of constricted passageways into a plurality of outlets into a zone of lower pressure to provide a plurality of closely adjacent extruded globules which then expand and adhere together to form strands which are then mechanically agitated to separate the strands into individual particles. U.S. Pat. No. 3,003,193 teaches intermittent extrusion of resin through a die with a non-reactive fluid between the intermittent resin particles as an ejection fluid.
Incorporation of the blowing agent into the extruder has been disclosed in Collins, U.S. Pat. No. 3,250,834. Collins compressed the heated polystyrene prior to introduction of the blowing agent and cooled the mixture thereafter extruding the mixture into strands which are later cut to pellets.
The problem of temperature control while cooling the extrusion passing through the die, as causing blockage of the die, is dealt with in UK 948,200 (Platz et al.) and Canadian Patent 682,464 (Schroeder et al.). Platz et al. make use of a die either made from or lined with polytetrafluoroethylene having openings of 2 to 8 mm in diameter. Schroeder et al. on the other hand deal with this problem by regulating the temperature of the composition entering the die to assure free flow and cooling the temperature almost immediately as it exits the die.
Other processes for the production of expandable resin compositions are disclosed in U.S. Pat. No. 3,026,273 (Engles), U.S. Pat. No. 3,026,272 (Rubens et al.), U.S. Pat. No. 3,121,132 (Bene) and U.S. Pat. No. 3,026,274 (McKillan et al.). These disclosures deal with the pre-expansion of the composition. Each of these processes contemplates the production of strands which are cooled "almost immediately" after leaving the die, some under increased pressure, before chopping the strand into pellets.
The process of underwater pelletizing of polymers with an adjustable blade for varying pellet size is disclosed in Smith, U.S. Pat. No. 4,500,271.
In U.S. Pat. No. 3,003,193, a process for the production of beads of thermoplastic polymers is disclosed where extrusion through a twin-screw extruder is employed to force the resin through a die under continuous pressure. The die is contacted on its face by a cutting means and thereafter a fluid which carries the globules to a zone of lower pressure where the fluid boils or evaporates and the globule is removed from the liquid and cooled to a bead. Although the patent teaches the production of uniform beads, bead size is limited in its minimum size.
It has now been discovered that foamable and expandable, elastomeric low density compositions and beads derived therefrom can be produced from compositions comprising a major amount of a block copolymer comprised of an alkenyl aromatic resin and a conjugated diene and a minor amount of one or more other thermoplastic resins.
A new method of making expandable compositions and foams has also been discovered. The method involves subjecting a bead or pellet of a polymeric composition in a dry state to a blowing agent under conditions of increased temperature and pressure for a time sufficient to obtain an expandable or foamable composition comprised of the bead or pellet having an amount of blowing agent impregnated therein.